The present invention relates generally to golf clubs and, more particularly, to composite golf club shafts and methods of manufacturing the same.
With the advent of composite golf club shafts, it has become much easier to tailor the design of a golf club to the needs of a particular player or particular shot. For example, for longer shots and lower numbered irons, or woods, it is often desirable to use a more flexible shaft. Whereas, for shorter shots and higher numbered irons, it is often more desirable to use a stiffer shaft. Such design goals may be achieved, for example, through the use of additional layers of composite fiber in shorter shafted clubs and through the use of fewer layers of fiber in longer clubs. Such design goals may also be achieved by varying the orientation of the layers of composite fiber that make up a shaft. For example, to add stiffness to a club shaft it may be desirable to utilize several layers of composite fiber that run parallel to the longitudinal axis of the shaft, whereas to enhance the flexibility of a shaft it may be desirable to utilize several layers of composite fiber which are offset to a substantial degree, for example, +/-45.degree. or more, from the longitudinal axis.
Those skilled in the art will appreciate that each layer of composite fiber may be formed using a "pre-preg" composite sheet, and that pre-preg composite sheets may be manufactured by pulling strands of fiber, for example, carbon or glass fiber, through a resin solution and allowing the resin to partially cure. Exemplary resins or "binding matrices" may include, for example, thermoset epoxy resins and thermoplastic resins. Alternatively, pre-preg sheets may be manufactured by pulling a fabric or weave of composite fabric through a resin solution and allowing the resin to partially cure. In either case, once the resin is partially cured or "staged," the resin holds the fibers together such that the fibers form a malleable sheet.
Similarly, selected regions of a club shaft may be reinforced through the provision of additional layers of composite fiber and by varying the direction of the composite fibers that may be found in a given layer. Indeed, it is quite common to provide additional layers of composite fiber in the tip region of a shaft to increase the torsional rigidity of the tip region and to insure that the tip region will not be damaged when a club head affixed thereto contacts a ball.
Finally, it has been found that by providing additional layers of composite fiber or by using pre-preg composite sheets having a weighting agent, such as iron, copper or tungsten powder, distributed therein, it is possible to adjust the overall weight, swing weight and balance point of a given shaft with a fairly reasonable degree of precision. Thus, it is possible conventionally to increase the overall weight of shorter club shafts through the use of additional fiber layers or through the use of weighted fiber sheets. Similarly, balance points and swing weights may be adjusted through the selective placement of additional fiber layers or weighted fiber sheets within a shaft.
There are, however, substantial drawbacks that are encountered when conventional techniques are used to adjust to any significant degree the overall weight, swing weight or balance point of a club shaft. For example, the addition of each layer of fiber alters to a significant degree the torsional and longitudinal flexibility characteristics of the shaft. The use of additional layers of fiber may also add substantially to the thickness of the walls of a shaft resulting in a dissimilarity in shaft wall thicknesses within a set. The reason for this is that many layers of fiber will generally be required to achieve a significant increase in shaft mass. It follows that, when conventional methods are used, it can be quite difficult to adjust the weight or distribution of mass within a golf club shaft without also varying to a significant degree the flexibility characteristics of the shaft.